Paint spray booths are utilized in the high rate of production paint finishing of automobiles, trucks and other equipment to provide controlled environment conditions and to confine and eliminate the overspray paint solids. The vehicle bodies or other parts to be painted are moved through and into the paint spray booth enclosures where workers man spray guns or automated equipment is caused to apply the paint to the vehicle bodies. The design of modern industrial paint spray booths presents considerable difficulties due to energy, environmental and working condition requirements which have become more severe in recent years.
In order to remove paint overspray solids and solvents from the air, large volume extraction fans have traditionally been used which exhaust the air to the atmosphere while drawing in a fresh air supply. In order to eliminate the casual discharge of paint solids, in recent years highly efficient air washing units have been used to clean the air prior to its discharge. The relatively high volumes of air which are circulated in order to insure healthful working conditions require enormous energy in heating the air to a temperature compatible with the painting process as well as for the workers' comfort.
While the washing techniques have been successful in reducing the overspray solids contained in the air after passing through the paint spray bottles, the hydrocarbon solvents in gaseous form cannot of course be removed by the same process. The rigorous air pollution standards as to such pollutants have initiated a trend to the use of water-based paints, such as to eliminate the use of hydrocarbon solvents entirely to meet environmental restrictions placed on the discharge of such materials.
This use of water-based paints has complicated the problem of conditioning air supplied to the booth. That is, in order for proper application of water-based paints, the air in the booth must be within fairly limited ranges of both temperature and relative humidity.
In a typical installation, the air must be heated and humidified during cold weather conditions, and cooled and dehumidified during warm weather conditions. These processes have, by conventional practice, required extensive equipment and also large expenditures of energy in order to carry out the proper heating, cooling and adjustment of humidity conditions of the air supply.
For example, in warm weather operation, there would generally be some form of refrigeration unit utilized to chill brine circulated to a cooling coil to cool the incoming air to an appropriate dew point temperature, condensing out the necessary quantity of moisture to achieve the desired relative humidity at the higher temperature at which ultimately the air is to be delivered to the spray booth. In order to reheat the air to this temperature, a separate heating unit has been used to warm the air to this higher temperature. In addition, cooling towers were utilized in conjunction with the refrigeration unit in order to dissipate the heat absorbed from the incoming air. In heating the air, separate heaters are used in order to heat the air to the appropriate supply temperature at which the air is required to enter the spray booth.
Since such systems currently are required to be through systems, i.e., all air directed into the spray booth is exhausted into the atmosphere without recirculation, the energy expended in the cooling and heating of the air is lost to the outside.
A basic problem in transferring heat into or out of the exhaust air is to avoid fouling of the heat transfer surfaces with the remaining air borne solids given the enormous air flow rates required, which has presented insuperable maintenance requirements.
While extraction of heat from the spray booth exhaust air has heretofore been carried out, difficulties are also presented in extracting significant heat energy from the relatively low temperature air. In those situations where the air is filtered by being washed in a water bath, the evaporative cooling not only reduces the temperature of the air, it also produces a moisture laden exhaust air. Attempts to extract heat which reduces the air temperature below freezing would result in freeze up of the heat exchanger.
Thus, it is broadly the object of the present invention to provide an arrangement for conditioning the air by heating, cooling, humidifying, or dehumidifying the incoming air supply as appropriate in order to provide supply air to such paint spray booths at a controlled dry-bulb temperature and relative humidity, which is highly efficient in terms of the use of energy and equipment in carrying out the conditioning of supply air during both warm and cold weather conditions.
It is a further object of the present invention to provide an arrangement for efficiently utilizing the exhausted air from the paint spray booth either as a source of heat or as a heat sink such as to recover at least partially the energy involved in either heating or cooling the air.
It is another object of the present invention to provide improvements in the technique of heat extraction such as to efficiently utilize the heat content of the exhaust air at various booths such as to improve the overall efficiency of the process as well as to reduce the complexity of the equipment required.